Researchers have uncovered new evidence that the first birds had four wings instead of two.

A team from China say they flew rather like a biplane - with wings on their legs providing a second boost.

The team analyzed 11 bird skeletons from between 150 and 100 million years ago, and believe they eventually shed the second set so they could use their legs more effectively.

Anchiornis huxley, a small, feathered, dinosaur, showing a second set of wings on its legs

4 winged birds: Arm and leg feathers in a fossil of the basal bird Sapeornis

Led by Xiaoting Zheng of China’s Linyi University, the authors of the new study analyzed 11 bird skeletons from between 150 and 100 million years ago, donated by various collectors to the Shandong Tianyu Museum of Nature.

The 11 birds come from five species and were relatively robust: larger than a crow but smaller than a turkey.

Recently, several fossils have shown that some dinosaurs had large
feathers on both fore limbs and hind limbs. But, until now, no examples
of this four-winged body plan have been described in birds -- or even in
their most recently extinct relatives.

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Modern birds have two types of feathers: vaned feathers that cover
the outside of the body, and the down feathers that grow underneath
them.

The Chinese team found that one type of vaned plumage, also known as pennaceous feathers,
was neatly preserved in skeletal fossils of these specimens, along each
creatures’ hind limbs.

These findings suggest that this four-winged condition preceded the
two-winged body plan, and that birds have gradually lost the feathers on
their hind limbs over evolutionary time.

However, some experts have questioned the results, and claim the feather may not have been used for flying, but as part of a mating display.

This ancient bird, one of 11 analysed by the China team, had stiff feathers on both arms and legs

Such a transition from four wings to two probably happened while the
birds were gaining more scales on their hind limbs and using them more
for terrestrial locomotion, according to the researchers.

In a related news article also published in Science, Michael Balter compares the ancient birds to biplanes in the early 1900s, which were later phased out in favor of faster monoplanes.

'A Chinese team presents dramatic new fossils suggesting that early birds went through a similar evolution,' he wrote, 'starting off with wings on both arms and legs and later adopting the arms-only, monoplane configuration.'

The study adds to existing theories that suggest the first birds flew with four wings.

Examination of a primitive bird fossil from the Archaeopteryx genus in 2004 revealed long feathers on the animal’s back and legs, which would have aided its gliding ability.

Two years later, another study of the crow-sized animal, which lived about 150 million years ago, reported that the prehistoric bird’s feathers resembled those on modern birds’ flight wings.